The present invention relates to a handpiece for dental or surgical use of the type comprising: a hollow shaft, which is rotatable around a longitudinal axis and mounted by bearings in a fixed tubular sheath; a clamp mounted in a front end of the hollow shaft and having a central channel intended to receive the shank of a removable tool, the clamp having axial arms arranged around said axis, each of which are provided with a gripping jaw in the central channel, and each arm being attached to an annular section of the clamp; a tightening mechanism supported by the hollow shaft and arranged to exert a centripetal force on a supporting surface of each arm of the clamp to grip the tool shank between the gripping jaws of the clamp; and a release mechanism supported at least partially by the fixed sheath and arranged to act on the tightening mechanism at least so as to loosen the clamp.
In the case of handpieces of this type the gripping jaws are usually located at an end of the clamp, which has conical outer supporting surfaces, on which an inside cone of a tubular piece engages to thus tighten the end of the clamp on the tool shank. For example, according to patent application FR 2 191 869, corresponding to U.S. Pat. No. 3,902,248, the clamp is mounted to slide in the hollow shaft, its gripping jaws and its outer supporting surfaces being located at its front end to cooperate with the front opening of the shaft, and the clamp is kept tight by a spring, which pulls axially to the rear.
In the case where the tool held by the handpiece has to withstand relatively high stresses, in particular with respect to the torque and axial forces, the grip of the clamp must be sufficiently tight. This requires a sufficiently high axial force of the tightening spring as well as high frictional forces on the conical surfaces, which convert this axial force into radial gripping forces. The release mechanism must be able to overcome these forces by applying a sufficient axial pressure on the sliding member biased by the tightening spring. This axial pressure is then transferred to the structure of the handpiece through at least one of the bearings supporting the rotatable shaft. Most of the time, as in the construction according to patent publication FR 2 191 869, this pressure is exerted forwards and thus stresses the bearings in the opposite direction to the axial stresses which the tool is subjected to. That is to say that the bearings must be designed to withstand axial stresses in both directions.
Moreover, a relatively strong tightening spring has a relatively high weight and this constitutes a particular disadvantage for a piece, which rotates very quickly and the centring of which cannot generally be assured with precision. The resulting vibrations increase with the weight of this spring.
Patent application FR 2 723 306, corresponding to U.S. Pat. No. 5,688,122, describes a device for gripping a dental tool in a head of an angle handpiece, comprising a clamp with elastic arms formed by cut-outs in a bushing also serving as a guide tube for the tool shank. In the resting position, the gripping jaws located on the arms of the clamp are at a distance from the axis that is smaller than the radius of the tool shank in such a way that this is then gripped solely by the elasticity of the arms. This device does not require an axial tightening spring, but the centrifugal force, however, tends to release the clamp.